Problem: Sharon bought a mixture of nuts that was made up of pecans, walnuts and cashews in a ratio by weight of $2:3:1$, respectively. If she bought $9$ pounds of nuts, how many pounds of walnuts were in the mixture? Express your answer as a decimal to the nearest tenth.
Since the pecan to walnut to cashew ratio is $2:3:1$, it follows that the walnut ratio to all of the nuts is equal to $\frac{3}{2+3+1} = \frac 12$. Thus, there were $\frac 12\times 9 = \boxed{4.5}$ pounds of walnuts in the mixture.